1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and, more particularly, to an output logic setting circuit which is arranged in a semiconductor integrated circuit to perform output logic setting.
2. Description of the Prior Art
An output logic setting circuit of this type includes a fuse to perform output logic setting. For example, such an output logic setting circuit is used in a constant-current generating circuit which has a fuse connected to the input terminal of a decoder for outputting trimming data so as to obtain necessary trimming data from the decoder by externally and selectively fusing the fuse.
FIG. 1 shows an output logic setting circuit arranged in a conventional semiconductor integrated circuit.
As shown in FIG. 1, this conventional circuit is constituted by a power-down reset circuit 100, an inverter circuit 101 having one terminal connected to the output stage of the power-down reset circuit 100 and the other terminal connected to an output terminal 10, and a selectively fusible fuse element 8 which is connected between the power-down reset circuit 100 and a ground potential V.sub.CC and is selectively rendered conductive or disconnected by external signal operation.
The power-down reset circuit 100 includes transistors 1 and 3 having gate terminals connected to a reset terminal 11, and a pull-up transistor 2 having a gate terminal connected to the ground potential V.sub.CC. The inverter circuit 101 includes transistors 4 and 5 having gate terminals connected to the output stage of the power-down reset circuit 100.
When the initial value in the output logic setting circuit having the above arrangement is to be changed, a fusing voltage is applied between a fusing pad 12 and the ground potential V.sub.CC to sever the fuse 8. The active-low power-down reset circuit 100 is then operated. With this operation, the input potential of the inverter circuit 101 on the subsequent stage is changed by the pull-up transistor 2 whose gate level is set to the ground voltage, thereby performing output logic setting.
In general, in an output logic setting circuit manufactured on a semiconductor substrate, a fuse element is made of a polysilicon layer formed on a field oxide film, and output logic setting is performed by selectively rendering the fuse element conductive or disconnecting it in accordance with external signal operation.
When, however, the output logic setting circuit shown in FIG. 1 is manufactured on a semiconductor integrated circuit for the above purpose, the resistance of the fuse element changes due to a change in quality over a long period of time since the manufacture of the circuit. In this case, the circuit may be returned to a connected state even if the fuse element is set to be in a disconnected state. As a result, the input voltage to the inverter circuit becomes unstable, and the output logic value may be inverted with respect to an expected value.